Our understanding of eukaryotic gene function and regulation remains rather limited. Analyses of gene systems in biochemical, developmental and genetic terms should help to provide information on this problem. The pteridine biosynthetic pathway in Drosophila melanogaster is proposed as a model system for the study of genes required for a common function. Currently under investigation is GTP cyclohydrolase (CH) which catalyzes the first step in the pathway. In the coming year, purification of the enzyme will be completed, anti-CH antibody will be produced and a physical characterization of the enzyme protein will be started. Study of the several mutants which have reduced CH activity will be continued. Characterization of mutant proteins will identify structural lesions which will be mapped to identify the structural gene (s). Mutagenesis programs will be started to obtain more mutant alleles. Developmental profiles and tissue localization of CH in wild-type and mutants will be determined and compared with the patterns of other pteridine biosynthetic enzymes.